“How dare you do it this way!”
So, as a matter of principle we need to stop acting as if resources are free. Also if the author couldn't even get that simple HTML detail right then it potentially says something about the quality of his site's information. Perhaps.
Perhaps is doing some heavy lifting here. I take it from the fact that the lectures are on Functional Programming that OP might not be a front-end developer and might not have the level of care needed for front end to compress the images in a way that renders nicely. So,
Perhaps you ought not judge someone releasing their free teaching materials for a backend class on their front end development, when the site is a centered <div> with purely static elements.
Perhaps you ought to slow down and do some critical thinking before being critical of others.
Thats not enough. It was completely bloody unnecessary to waste enormous amounts of co2.
And critical thinking should lead to the realization that not many people have to solve the - albeit small - mistake of an individual via Cloudflare or similar, but rather someone should do it correctly right away. And if he has no knowledge of frontend, he should not do things he has no knowledge of.